1. Field of the Invention
The present invention relates to a display apparatus and, more particularly, to a display apparatus which is suitable as a projecting apparatus such as a video projector or the like in which an image which was projected and formed at an angle by projecting means is observed from a space on the side opposite to the projecting means.
2. Related Background Art
Hitherto, in the projecting apparatus such as a video projector or the like, there has widely been used what is called a back projecting type screen in which an image from projecting means is formed onto a screen and the projection image is observed from a space on the side opposite to the projecting side, that is, the back side.
FIG. 1A schematically shows an example of a projecting system using a conventional back projecting type screen.
In the diagram, reference numeral 31 denotes a transmitting type screen having a Fresnel lens, 33 indicates a CRT, and 34 a projecting lens. The screen 31 has a size of a diagonal length of, for instance, 50 inches (1100 mm.times.600 mm). A projection image by the projection lens 34 is formed onto the screen 31. FIG. 1B shows an example of a back projecting type display apparatus in which such a projecting system is enclosed in a cabinet. Reference numeral 35 and 36 denote mirrors and 37 indicates a cabinet.
In FIG. 1A, the center of the pupil of the projecting lens 34 exists at a position 130 mm away from from the CRT 33 and 1500 mm away from the screen 31. An incident angle of a light flux which enters the outermost peripheral portion on the diagonal line of the screen 31 from the center of the pupil of the projecting lens 34 to the screen 31 is set to about 23.degree.. The light flux transmitted through the screen 31 is focused to a position corresponding to the distance which is about eight times larger than the height (600 mm) of the screen 31 by the Fresnel lens on the screen 31.
As examples of the screens 31 shown in FIGS. 1A and 1B, there have been known screens as shown in FIGS. 2A and 2B each of which illustrates a cross sectional view of the central portion of the screen. A screen 41 of FIG. 2A is constructed such that the light flux incident side is set to a Fresnel lens surface 41a and the emitting side is set to a lenticular lens surface 41b. FIG. 3A shows a light path of the light flux which enters the outermost peripheral portion of the screen 41 at an angle of 23.degree. to an optical axis (indicated by a chain line in FIG. 1A) of the projecting lens 34. In this case, a refractive index of the material of the screen 41 is set to 1.5 and the emitting side of the screen 41 is shown as a plane surface. In the lower portion of FIG. 3A, progressive angles .alpha. (angles to the optical axis, that is, the horizontal direction of the projecting lens 34) of the light flux before and after the transmission of each surface are shown.
Assuming that a cross sectional shape of prisms which extend like an arc and form the peripheral portion of the Fresnel lens surface 41a is as shown in FIG. 3A, an incident angle of the prism to a Fresnel lens effective surface 41c is set to 65.degree. and a transmission factor at this time is set to 88%. An incident angle to the emitting surface is set to -5.degree. (an angle in the case of the clockwise direction when it is measured from the optical axis is set to a minus value). A transmission factor at the emitting surface at this time is set to 96%.
Further, the light flux which enters a non-effective surface 41d of the Fresnel lens surface 41a is lost. A loss ratio Q of the lost light to an incident light amount is expressed as follows. EQU Q=tan .alpha..multidot.tan .theta.
In the example of FIG. 3A, Q is set to about 38%.
Thus, in the case of the projecting system of the vertical incident type using the screen 41 shown in FIG. 2A, a transmission factor T.sub.1 in the outermost peripheral portion of the screen 41 is set to ##EQU1## Therefore, the light amount is reduced by about 43% as compared with the transmission factor of 92% in the central portion of the screen.
On the other hand, a screen 42 shown in FIG. 2B is constructed to prevent the reduction in light amount in the peripheral portion. In FIG. 2B, reference numeral 43 denotes a translucent sheet in which the incident side, namely, the back side is set to a plane surface and the emitting side, that is, the observing side is set to a Fresnel lens surface 43a, and 44 indicates a translucent sheet in which the incident side is set to a plane surface and the emitting side is set to a lenticular lens surface 44a.
FIG. 3B shows a light path of the light flux which enters the outermost peripheral portion of the sheet 43 under the conditions similar to those in FIG. 3A. Assuming that prisms which form the peripheral portion of the Fresnel lens surface 43a have a cross sectional shape as shown in the diagram, the light flux shown by A is not lost at a non-effective surface 43b but transmits the Fresnel lens surface 43a. Therefore, in this case, the light flux is lost by only the amount of light reflected by the surface. Thus, a transmission factor in the outermost peripheral portion is about 90% and is almost equal to that in the central portion of the screen 42.
The above description relates to the projecting system of the vertical incident type in which the optical axis of the projecting lens 34 vertically crosses at the center of the screen surface. On the other hand, in order to miniaturize the whole system (particularly, to reduce the depth), as shown by broken lines in FIG. 1A, it is demanded to realize a projecting system of the type in which an image from a CRT 33a is projected onto a screen 31a at an angle through a projection lens 34a, that is, of the oblique incident type in which the optical axis of the projection lens obliquely crosses the screen surface.
A back projecting type display apparatus of such a type is constructed as shown in FIG. 1C. In the diagram, reference numerals 35a and 36a denote mirrors and 37a indicates a cabinet. An image displayed on the screen of the CRT 33a is obliquely projected onto the screen 31a from the right upper direction. In FIG. 1C, although the image projecting direction is different from that of the system shown by broken lines in FIG. 1A, both of them are optically substantially equal.
With the above construction, a depth L of the cabinet 37a can be reduced as compared with that in the case of the vertical incident type.
However, in the case of the construction of FIG. 1C, the projection image light which is emitted from the screen 31a to the left observing side is emitted on the side which is lower than the horizontal direction by only an angle .theta..sub.0. Thus, the image becomes dark for an observer who observes from the front side of the screen 31a. Therefore, in order to emit the projection image light from the screen 31a in the horizontal direction, it is necessary to construct a screen having an eccentric Fresnel lens as shown in FIG. 1D.
However, in the oblique incident type, there is a case where an incident angle of the light flux which enters the outermost peripheral portion (particularly, the lower peripheral portion in FIG. 1C) of the screen 31a is set to 45.degree.. In this case, even if the screen 42 shown in FIG. 2B which can preferably function in the vertical incident type is used, an incident angle of the light flux to the second surface (that is, the Fresnel lens surface 43a in FIG. 3B) of the screen 42 in the outermost peripheral portion (in particular, the lower portion) reaches an angle near the total reflection angle or exceeds the total reflection angle, so that a peripheral light amount suddenly decreases. Particularly, in a construction in which three CRTs of red, green, and blue are arranged as a projector for a color image, incident angles to the screen differ every color and a color shift is caused. The color shift is further amplified due to a sudden increase in reflectance at the foregoing surface.
As means for solving the above problem, there is considered a method whereby as shown in, for instance, FIG. 1E, the power is distributed by using two Fresnel lens surfaces on each of which a number of prisms are formed, thereby preventing a peripheral light amount from being suddenly reduced even if an incident angle of the light flux to the peripheral portion of the screen increases. However, according to such a construction, there is a fear such that a moire or a multiple image occurs due to a repetitive periodic structure of a plurality of prism groups and the observation of the projection image is obstructed.
When solving the above problems, it is necessary to also consider the following points. The projection lens 34 (34a) of the projector shown in FIG. 1A has a limited pupil diameter. Therefore, an incident angle of the light flux which enters a certain point on the screen has a limited extent around the principal ray as a center. On the other hand, in the construction in which three projection lenses for red, green, and blue are arranged, since each lens has a limited pupil diameter, a range of the incident angle is further extended.